Low temperature liquefied gas tank of a membrane type

ABSTRACT

A tank of membrane type for containing low temperature liquefied gases such as petroleum gases, comprising an outer vessel of a rigid structure, an inner vessel made of a membrane or thin sheet for containing low temperature liquefied gases, and a heat insulating intermediate layer disposed between the outer and inner vessels, wherein the improvement resides in that a secondary inner vessel made of a membrane or thin sheet is provided between the heat insulating intermediate layer and the first mentioned or primary inner vessel as a secondary barrier wall which prevents the liquefied gases which may leak out of the primary inner vessel due to breakage thereof from affecting the heat insulating intermediate layer and the outer vessel.

United States Patent [191 Yamamoto Dec. 25, 1973 LOW TEMPERATURE LXQUEFIED GAS TANK OF A MEMBRANE TYPE [75] Inventor: Katsuro Yamamoto, Tokyo, Japan [73] Assignee: Bridgestone Liquefied Gas Company, Ltd., Tokyo, Japan [22] Filed: Oct. 22, 1971 [21] App]. No.: 191,945

[30] Foreign Application Priority Data 220/85 UR, 85 US, 9 B, 9 D,.3; 114/74 A [56] References Cited 3,647,103 3/1972 Berman 220/3 3,319,430 5/1967 Small 220/9 LG FOREIGN PATENTS OR APPLICATIONS 1,035,957 1966 Great Britain 220/9 LG Primary Examiner-William T. Dixson, Jr. Assistant ExaminerA1lan N. Shoap Attorney-Woodhams, Blanchard et al.

[ ABSTRACT A tank of membrane type for containing low temperature liquefied gases such as petroleum gases, comprising an outer vessel of a rigid structure, 'an inner vessel made of a membrane or thin sheet for containing low temperature liquefied gases, and a heat insulating intermediate layer disposed between the outer and inner vessels, wherein the improvement resides in that a secondary inner vessel made of a membrane or thin sheet is provided between the heat insulating intermediate UNITED STATES PATENTS layer and the first mentioned or primary inner vessel 646,459 4/1900 Place 220/9 LG as a secondary barrier wall which prevents the lique- 3,282,459 11/1966 Wilson 220/3 fied gases which may leak out of the primary inner 3562-936 2/1971 220/9 LG vessel due to breakage thereof from affecting the heat 3319x431 5/1967 Clarke 220/9 LG insulating intermediate layer and the outer vessel. 1,424,604 8/1922 Weber 220/9 LG 3,666,132 5/1972 Yamamoto et a1 220/9 LG 7 Claims, 4 Drawing Figures 2 33 5b 8 5 IO 7 p \l 4' R s 56 R2 I l 1 l m 11 II F 1 4 &8 \x

PATENTED DEC 2 5 I975 FIG. I

0 I NVENTOR man/m FAA/441070 J 3 BY flHZW, KM 0% 2 ATTORNEYS BACKGROUND OF THE. INVENTION 1. Field of the Invention This invention relates to a tank of a membrane type for containing low temperature liquefied gases such as petroleum gases, which are gaseous at normal temperature, but which are liquefied at normal pressure and low temperature.

2. Description of the Prior Art A conventional tank for the abovementioned purpose is generally composed of an outer vessel of a pressure-resisting structure, a heat insulating layer provided on the inside of the outer vessel, and an inner vessel made of a membrane or thin sheet and adapted tobe flexibly deformed by the internal pressure exerted by the liquefied gases contained therein so as to come in close contact with the inside surface of the heat insulating layer, whereby the internal pressure is supported by the outer vessel through the heat insulating intermediate layer.

In the conventional tank of the abovementioned type, it is generally required that in addition to the abovementioned structure a secondary barrier wall is provided to temporarily prevent leakage of liquefied gases when the liquid-tightness of the inner vessel has been broken. The secondary barrier wall must not only be liquid-tight but also withstand hydraulic pressure and low temperature, and therefore, the secondary barrier wall requires expensive materials and a complicated design. Thus, they provision of the secondary barrier wall increases the overall manufacturing cost of the tank.

SUMMARY OF THE INVENTION Accordingly, it is the main object of this invention to dispense with the secondary barrier wall in the tank of the conventional structure and to provide an improved structure for a low temperature liquefied gas tank of a membrane type, wherein leakage of liquefied gases due to an unexpected loss 'of liquid-tightness of the inner membranous vessel is effectively prevented by a simple and less expensive structure.

The abovementioned object is accomplished, according to this invention, by a low temperature liquefied gas tank of a membrane type comprising an outer vessel of a rigid structure, an inner vessel made of a membrane for containing low temperature liquefied gases, and a heat insulating intermediate layer disposed between the outer and inner vessels, characterized in that a secondary inner vessel made of a membrane is provided between the heat insulating intermediate layer and the first mentioned inner vessel.

According to the above mentioned feature of this invention, the first mentioned inner vessel, which is called a primary inner vessel hereinunder, is wrapped by the secondary inner vessel in close contact therewith in a manner of providing a dual-walled membranous vessel for containing low temperature liquefied gases therein, while the secondary inner vessel also performs the role of a secondary barrier wall, whereby it is unnecessary to provide the conventional secondary barrier wall of a relatively complicated and expensive structure incorporated in the heat insulating layer.

According to an additional feature of this invention, the secondary inner vessel is made of a membrane or thin sheet which has a smaller thickness than the membrane forming the primary inner vessel, while the radius of curvature at'the curved edge portions of the secondary inner vessel is designed to be smaller than that of the primary inner vessel. By this arrangement of the radius of curvature, the secondary inner vessel, which may be made of a relatively thin membrane or film, is released from being applied a high hoop tension at the edge portions thereof when the internal pressure in the primary inner vessel is applied to the secondary inner vessel because of breakage of the primary inner vessel.

Thus, high reliability of the tank is obtained even when a relatively thin membrane or film is used as the secondary inner vessel.

According to still another feature of this invention, the pressure in the space formed between the primary and secondary inner vessels is controlled to be not lower than the pressure prevailing at the outside of the secondary inner vessel and to be not higher than the pressure in the primary inner vessel. By-this arrangement of the pressure difference, it is ensured that the curved edge portions of the primary and secondary inner vessels are constantly extended convexly outward and it is avoided that there occurs buckling of the edge portions due to such a reversed pressure gradient that would act to flatten the curved edge portions.

According to a further feature of this invention, the space formed between the primary and secondary inner vessels and the space formed between the secondary inner vessel and the outer vessel are respectively communicated through gas pipes to gas analyzing means, whereby leakage of the primary inner vessel is readily detected by analyzing the atmosphere filling the space formed between the primary and secondary inner vessels, while leakage of the secondary inner vessel is readily detected by analyzing the atmosphere filling the space formed between the secondary inner vessel and the outer vessel.

According to a still further feature of this invention, overhanging roof portions of the primary and secondary inner vessels are supported by a plurality of cantilevers extending from peripheral portions of a trunk of the tank. In a low temperature liquefied gas tank of a membrane type, it is generally required that the membranous inner vessel is applied as little restriction as possible so that the inner vessel can be more freely deformed according to variations of temperature and/or load, while it is required that the inner vessel is properly supported from collapsing when the inside of the inner vessel is laid under normal presure, such as at the time of construction or maintenance. By the provision of the cantilevers as mentioned above, it is accomplished that the membranous primary and secondary inner vessels are effectively supported from collapsing while they are left free from any substantial restriction in case of deformation thereof due to variations of temperature and/or load.

BRIEF DESCRIPTION OF THE DRAWING This invention will now be described more particularly with reference to the accompanying drawing which shows an embodiment of this invention applied to a low temperature liquefied gas tank of a tanker.

In the drawing,

FIG. 1 is a cross section of a part the hull of a tanker constituting a low temperature liquefied gas tank;

FIG. 2 is a section along line II-II in FIG. 1; and

FIG. 3 is a section along line III-Ill in FIG. 1. FIG. 4 isa section, on an enlarged scale, along the line IV-IV in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a dual-walled hull-l including a rigid inner wall la provides arigid outer vessel of a low temperature liquefied gas tank incorporated in the tanker. At the inside of the inner wall la, there is provided a heat insulating layer 2 of a pressure resisting structure. Further .at the inside of the heat insulating layer 2, there is provided a membranous vessel 3a as the secondary inner vessel, and still further at the inside of the membranous vessel 30, there is provided another membranous vessel 3 as the primary inner vessel.

The heat insulating layer 2 may be made of a material having by itself pressure resisting characteristic, such as rigid polyurethane foam, or may be made of frames such as wooden frames defining a space which is filled with heat insulating materials such as granular pearlite, etc. The inner vessels 3 and 3a are respectively formed of a membrane or thin sheet of low temperature resisting material such as nickel steel, stainless steel or aluminum. When, for example, the primary inner vessel 3 has a thickness of about 4 to 6 mm, the thickness of the secondary inner vessel 3a is designed to be about 1 mm. Thus, the secondary inner vessel 3a is formed much thinner than the primary inner vessel 3 (See FIG. 4)

About at the center of a roof portion of the tank, there is provided a trunk 5 having a rigid structure reinforced by brackets 4. The trunk 5 is gas-tightly connected with the primary and secondary inner vessels 3, 30 at its flanged portion 5a provided adjacent the lower end thereof. The space formed between the primary and secondary inner vessels is also gas-tightly closed at the portion where they are connected with the flanged portion 5a.

The trunk 5 is mounted to the dual-walled hull l by the flanged portion 5a and another flanged portion 5b via mounting pieces 6 and 7 made ofa rigid heat insulating material. An annular space 8 formed between the hull l and the trunk 5 is filled with a heat insulating material so that the low temperature in the inner vessel does not directly affect the hull of the tanker.

Loading and unloading pipes, gas pipes, etc. (not shown) to be introduced into the primary inner vessel 3 are all passed through the trunk 5. The trunk is closed by a cover (not shown), through which the abovementioned pipes are gas-tightly penetrating.

A plurality of cantilevers nine are mounted at lower end portions of the trunk 5 along the periphery thereof so that the cantilevers extend radially outwardly around the trunk. By these cantilevers are supported the roof portions of the primary and secondary inner vessels 3, 3a from collapsing due to their gravity when the inside of the primary inner vessel 3 is laid under normal pressure at non-loading condition.

The gas-tight space formed between the primary and secondary inner vessels 3, 3a is in communication with a gas pipe 10, and the space formed between the secondary inner vessel 3a and the rigid hull inner wall la is in communication with a gas pipe 11. These two pipes can be communicated with each other by a connecting pipe 13 including a valve 12 therein, and are respectively connected to pumps (not shown). These gas pipes serve to extract gas samples for analyzing the ing to the following formulae:

curved edge portions of the primary and secondary inner vessels is determined based upon the maximum internal pressure expected, the thickness of the respective inner vessels, and the allowable hoop stress of the materials forming the respective inner vessels accord- 0' PRg/tg where,

R radius of curvature at a curved edge portion of the primary inner vessel 3 R radius of curvature at a curved edge portion of the secondary inner vessel 3a P maximum internal pressure expected t,; thickness of the primary inner vessel t thickness of the secondary inner vessel 0-,: allowable hoop stress of the material forming the primary inner vessel 3 0' allowable hoop stress of the material forming the secondary inner vessel 3a The maximum internal pressure P expected, to be applied to the primary inner vessel 3 is also used in Formula 2 for the secondary inner vessel 3a in view of the fact that the secondary inner vessel is exposed to such an internal pressure ifa leakage has occurred in the primary inner vessel 3.

If it is assumed that the primary and secondary inner vessels 3, 3a are made of a same material and the thickness of the latter is one fourth of that of the former, 0-, and 0 are ofa same value, and therefore, the condition of R =4R is obtained from Formulae l and 2. Therefore, the radius of curvature at a curved edge portion of the secondary inner vessel is designed to be one fourth of thatof the primary inner vessel.

FIG. 2 shows a section along line Il--ll in FIG. 1 and actually shows an edge portion of the tank extending in vertical direction. Also in this case, the curved edge portions of the primary and secondary inner vessels 3, 3a are so formed that the radius of curvature R of the secondary inner vessel is smaller than the radius of curvature R of the primary inner vessel.

However, as shown in FIGS. 1 and 3, the latter showing a section along line IlI-lll in FIG. 1, the curved edge portions of the primary and secondary inner vessels extending along the bottom edge portion of the tank are formed to have a substantially same radius of curvature, because at these portions the two inner vessels are both pressed by the liquid load contained in the primary inner vessel to come in tight contact with the inside surface of the heat insulating layer 2.

The pressure in the space formed between the primary and secondary inner vessels 3, 3a is adjusted to be not lower than the pressure in the space formed between the secondary inner vessel 3a and the hull inner 1a and to be not higher than the pressure in the primary inner vessel 3 through the gas pipes 10 and l 1, whereby it is positively avoided that the curved edge portion of the secondary inner vessel made of a relatively thin membrane or film and having a smaller radius of curvature is pressed upon the curved edge portion of the primary inner vessel having a larger radius of curvature and is thereby caused to buckle.

A convenient method of keeping the pressure in the space formed between the primary and secondary inner vessels 3, 3a constantly not to be lower than the pressure in the space formed between the secondary inner vessel 3a and the hull inner wall la is to put the pipes and 11 in communication with each other through the pipe 13 by opening the valve 12. In this case, the valve 12 must of course be closed when the gases in the abovementioned spaces are to be analyzed through the pipes 10 and 11.

The space formed between the secondary inner vessel 3a and the hull inner wall la including the heat insulating layer 2 may preferably be charged with nitrogen gas or other inert gas to reduce the danger of ignition of the combustible gases in case that they leak out of the primary inner vessel into the space. By charging the space formed between the primary and secondary inner vessels 3, 3a with a gas other than nitrogen or inert gas, such as for example ammonia, it is possible to detect leakage of the secondary inner vessel 3a by analyzing the gas taken out through the pipe 11.

When it was detected, as a result of the gas analysis, that the gases contained in the primary inner vessel 3 are mixed in the gas taken out through the pipe 10 from the space formed between the primary and secondary inner vessels 3, 3a, it is guessed that there is a leakage of the primary inner vessel 3. However, since in this case the secondary inner vessel 30 serves as a secondary barrier wall which has sufficient fluid-tightness and durability, the tank requires no urgent countermeasure.

From the foregoing, it will be appreciated that the secondary inner vessel 3a according to this invention can be constructed much easier and at much lower cost than the conventional secondary barrier wall, and yet the secondary inner vessel provides very high reliability of a low temperature liquefied gas tank of a membrane type even under severe operating condition imposed on such tanks.

I claim:

1. A membrane-type tank for storing lowtemperature liquefied gas, comprising: a rigid outer vessel; a heat-insulating, pressure-resistant intermedi ate layer provided on the inside of said outer vessel; a

double-walled inner vessel disposed inside said intermediate layer and including a primary inner vessel made of a flexible membrane and adapted to contain low temperature liquefied gas therein and a secondary inner vessel made of a flexible membrane and enclosing said primary inner vessel therein, said membranes being made of low-temperature resisting metal, said double-walled inner vessel being supported so as to be flexibly deformed by the internal pressure exerted by the liquefied gas contained therein to come into close contact with the inside surface of the heat-insulating layer wherein said primary and secondary inner vessels are in close contact with each other over substantially the entire area thereof, whereby the internal pressure is supported by the outer vessel through the heatinsulating intermediate layer, the membrane forming the secondary inner vessel having a smaller wall thickness than the membrane forming the primary inner vessel, and the radius of curvature at curved edge portions of the secondary inner vessel being smaller than the ra dius of curvature of corresponding edge portions of the primary vessel.

2. A tank according to claim 1, wherein the pressure in the space formed between the primary and secondary inner vessels is controlled to be not lower than the pressure in the space formed between the secondary inner vessel and the outer vessel and to not be higher than the pressure in the primary inner vessel.

3. A tank according to claim 1, wherein the space formed between the primary and secondary inner vessels and the space formed between the secondary inner vessel and the outer vessel are respectively communicated through gas pipes to gas analyzing means.

4. A tank according to claim 3, wherein said two gas pipes are connected with each other by a pipe having a cut-off valve therein.

5. A tank according to claim 1, wherein overhanging roofportions of the primary and secondary inner vessels are supported by a plurality of cantilevers extending from peripheral portions of a trunk of the tank.

6. A tank according to claim 1, wherein the space formed between the outer vessel and the secondary inner vessel is charged with inert gas.

7. A tank according to claim 1, wherein the space formed between the primary and secondary inner vessels is charged with a gas other than the gas charged in the space formed between the outer vessel and the secondary inner vessel. 

1. A membrane-type tank for storing low-temperature liquefied gas, comprising: a rigid outer vessel; a heat-insulating, pressure-resistant intermediate layer provided on the inside of said outer vessel; a double-walled inner vessel disposed inside said intermediate layer and including a primary inner vessel made of a flexible membrane and adapted to contain low temperature liquefied gas therein and a secondary inner vessel made of a flexible membrane and enclosing said primary inner vessel therein, said membranes being made of low-temperature resisting metal, said double-walled inner vessel being supported so as to be flexibly deformed by the internal pressure exerted by the liquefied gas contained therein to come into close contact with the inside surface of the heat-insulating layer wherein said primary and secondary inner vessels are in close contact with each other over substantially the entire area thereof, whereby the internal pressure is supported by the outer vessel through the heat-insulating intermediate layer, the membrane forming the secondary inner vessel having a smaller wall thickness than the membrane forming the primary inner vessel, and the radius of curvature at curved edge portions of the secondary inner vessel being smaller than the radius of curvature of corresponding edge portions of the primary vessel.
 2. A tank according to claim 1, wherein the pressure in the space formed between the primary and secondary inner vessels is controlled to be not lower than the pressure in the space formed between the secondary inner vessel and the outer vessel and to not be higher than the pressure in the primary inner vessel.
 3. A tank according to claim 1, wherein the space formed between the primary and secondary inner vessels and the space formed between the secondary inner vessel and the outer vessel are respectively communicated through gas pipes to gas analyzing means.
 4. A tank according to claim 3, wherein said two gas pipes are connected with each other by a pipe having a cut-off valve therein.
 5. A tank according to claim 1, wherein overhanging roof portions of the primary and secondary inner vessels are supported by a plurality of cantilevers extending from peripheral portions of a trunk of the tank.
 6. A tank according to claim 1, wherein the space formed between the outer vessel and the secondary inner vessel is charged with inert gas.
 7. A tank according to claim 1, wherein the space formed between the primary and secondary inner vessels is charged with a gas other than the gas charged in the space formed between the outer vessel and the secondary inner vessel. 